The present invention relates to a display device in which pixels are controlled by respective mirrors.
U.S. Pat. No. 5,517,347 discloses a directly viewable display device, that is it does not project an image onto a screen, in which the appearance of each pixel is controlled by the state of a respective mirror. However, it suffers from the problem that the image has a smaller area than the display device because light enters and leaves the device obliquely.
It is an object of the present invention to overcome the afore-mentioned problem of a prior art display device and provide a display device, in which each pixel is controlled by the state of a respective mirror, that can be viewed face on.
According to the present invention, there is provided a display device comprising a front face, means defining an array of directly viewable pixels and a respective mirror for each pixel, the state of which controls the appearance of the pixel, wherein light beams are reflected by said mirrors through diffusing means to a viewing position, emerging from the front face in a direction substantially perpendicular thereto, the diffusing means expanding said beams for viewing.
Preferably, light source means is provided for supplying pixel illuminating light to the mirrors and the light paths from the light source means to the mirrors are substantially equal in length.
Preferably, a display device according to the present invention includes a light source layer and a control layer containing the mirrors, wherein the light source layer and the control layer are substantially co-extensive.
The preferred lighting arrangements of the present invention overcome the subsidiary problem of non-uniform pixel illumination that arises with the side-lit arrangement disclosed in U.S. Pat. No. 5,517,347.
Preferably, the control layer includes focusing means, associated with each mirror, for reducing the cross-sectional areas of light beams that are reflected by the mirrors to be less that the cross-sectional areas of the visible pixels and a diffusion layer is provided for expanding the beams for viewing. The control layer may sandwiched between the light source layer and diffusion layer or the light source layer may sandwiched between the diffusion layer and the control layer.
Preferably, each pixel is defined by a cell, each cell comprising means defining a light path selectively extending between a light source and a viewing position. Each light path may selectively be interruptable by changing the state of the respective mirror.
Preferably, each mirror is tiltable between a first position in which its pixel is bright and a second position in which its pixel is dark. However, the mirror could be translated or have their reflectivities changed.
In one embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light through a lens to a tiltable mirror and then to a fixed mirror, which directs light received via the lens and the tiltable mirror through an aperture in the tilting mirror to the diffusing means and thence to a viewing position, the lens focusing light from the light source at a point between the fixed mirror and the viewing position. Conveniently, the lens comprises a fresnel lens.
In another embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light to a fixed concave parabolic mirror and then to a tiltable mirror which directs light in the light path to a viewing position through the diffusing means, the parabolic mirror focusing light from the light source to a point between the tiltable mirror and the viewing position.
The diffusing means may for instance comprise a hemispherical lens and a planoconcave lens.
Advantageously, the light path of at least one cell passes through a colour filter. In this way a colour image can be produced. Preferably, different colour filters are provided in different light paths. A full-colour display is most desirable and this can be achieved by providing different colour filters, e.g. red, green and blue, in different light paths.
In a further embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light via a convex lens to a tiltable mirror which directs light in the light path to a viewing position through the diffusing means, the lens focusing light from the light source to a point between the tiltable mirror and the viewing position. Conveniently, the lens comprises a fresnel lens.
In a yet further embodiment, each cell is provided with a source of a narrow beam of collimated light and the light path, when uninterrupted, extends from the source of light via a fixed planar mirror to a tiltable planar mirror which directs light in the light path to a viewing position through the diffusing means. Preferably, each cell is provided with a plurality of colour filters and the tiltable mirror is controllable to direct light in the light path selectively through the filters.
An electroluminescent polymer may be used as the light source.
The depth of the control layer is conveniently defined by a spacer structure, the spacer structure defining a matrix of cells.